Intestinal infections with protozoan parasites such as Entamoeba histolytica, Giardia lamblia, Cryptosporidia, and Isospora belli are frequently found in homosexual men within the United States, in individuals living in tropical and subtropical areas, and in travelers returning from highly endemic areas. Diagnosis of these organisms is often difficult and requires sequential stool examination by experienced personnel. As many as 50% of infected patients have no parasites demonstrable by a single stool examination, and require additioanl examinations for diagnosis. Enzyme-linked immunosorbent assays (ELISA) for the detection of pathogenic antigen in stool specimens offer the advantages of being rapid and easy to perform on large numbers of specimens, standardized without difficulty and adjunctive in epidemiologic studies of these infections. We have recently developed indirect double antibody ELISA systems which detect G. lamblia or E. histolytica in single stool specimens. Studies are presently under way for the detection of Cryptosporidia and Isospora in stool specimens by ELISA. For Giardia, stool specimens were positive by ELISA in 36 of 39 (92%) of patients with giardiasis; negative specimens came from patients with low number of parasites in the stool. In ten patients followed prospectively, stools became negative by ELISA after successful treatment. Stool specimens were positive by ELISA in 3 of 128 (2%) of patients without demonstrable Giardia in their stool, but who were at high risk for giardiasis due to IgA deficiency and whose diarrhea clinically responded to treatment for Giardia. For E. histolytica, 15 of 16 (94%) patients infected with E. histolytica were positively by ELISA. Ninety-eight (92%) of 106 specimens from patients without demonstrable E. histolytica in their stool were negative by ELISA. Coinfection with multiple other parasites did not appear to affect sensitivity or specificity. These ELISA tests are simple, sensitive and specific diagnostic tests which will be useful both in individuals and in large scale epidemiologic investigations. Present studies are under way to develop this test for the detection of Cryptosporidia and Isospora antigen from AIDS AND non-AIDS patients.